High Strength Seamless Steel Pipe for Machine Structure Use Superior in Toughness and Weldability, and Method of Production of The Same

ABSTRACT

A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass %, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further containing one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.

TECHNICAL FIELD

The present invention relates to seamless steel pipe suitable for machine structural members, in particular cylinders, bushing, booms, and other structural members and shafts and other machine members and a method of production of the same.

BACKGROUND ART

The machine parts used in automobiles and industrial machinery are made by forging and cutting steel bars into predetermined shapes, then heat treating them by patenting to impart predetermined mechanical properties. In recent years, for hollow shaped articles, steel pipes having the required mechanical properties have been used as the material to shorten the forging process and eliminate the heat treatment process to reduce costs in an increasing number of cases.

However, in general, steel pipe is more expensive than steel bars. In particular, seamless steel pipe is high in production costs, so even if using steel pipe as a material for hollow shaped articles, the effect in reducing costs sometimes cannot be said to be sufficient.

Therefore, provision of inexpensive steel pipe reduced in production costs is being studied. So-called non-patented steel pipe for machine part use and steel pipe for structural use eliminating the patenting heat treatment after hot pipe formation are being proposed (for example, see Japanese Patent Publication (A) No. 5-202447, Japanese Patent Publication (A) No. 10-130783, Japanese Patent Publication (A) No. 10-204571, Japanese Patent Publication (A) No. 10-324946, Japanese Patent Publication (A) No. 11-36017, Japanese Patent Publication (A) No. 2004-292857, and Japanese Patent Publication (A) No. 2001-247931).

However, the steel pipes described in Japanese Patent Publication (A) No. 5-202447, Japanese Patent Publication (A) No. 10-130783, Japanese Patent Publication (A) No. 10-204571, Japanese Patent Publication (A) No. 10-324946, Japanese Patent Publication (A) No. 11-36017, and Japanese Patent Publication (A) No. 2004-292857 all are high in amount of C and have large amounts of carbonitride-forming elements added to improve the hardenability and precipitation strengthening ability and obtain a predetermined strength.

For this reason, the alloy cost becomes higher and, further, preheating etc. become necessary for preventing cracking at the time of welding. There is the problem of impairment of productivity.

The method described in Japanese Patent Publication (A) No. 2001-247931 performs the hot rolling at the considerably low temperature of 600 to 750° C. to make the metallurgical structure finer and improve the strength. Low temperature rolling is a general technique in thick-gauge plate rolling, but if performing seamless rolling at a low temperature, there are the problems that contact with tools causes easy occurrence of imperfections or seizing etc., so in practice the scope of application is greatly limited.

Further, technology for hot rolling seamless steel pipe, then acceleratedly cooling it to improve the strength is being proposed (for example, see Japanese Patent No. 3503211 and Japanese Patent Publication (A) No. 7-41856). The method described in Japanese Patent No. 3503211 allows the inner surface of the steel pipe after final finishing rolling to cool, cools the outer surface from a temperature of the Ar₃ point or more to 500 to 400° C. by 10 to 60° C./s, then allows it to gradually cool. The method described in Japanese Patent Publication (A) No. 7-41856 comprises direct quenching or accelerated cooling as hot rolled.

However, these are oil well pipes. They are tempered after accelerated cooling, so the production cost is high. Weldability does not have to be considered, so 0.1% or more of carbon is contained. As opposed to this, among machine structure use steel pipe, steel pipes used for cylinders, bushings, etc. are often required to exhibit toughness and weldability and are preferably limited to amounts of carbon of less than 0.1%.

DISCLOSURE OF THE INVENTION

The present invention was made in consideration of the above situation. In particular, it provides seamless steel pipe suitable for machine structural member use for cylinders, bushings, booms, or other structural members and shafts or other machine members where high strength, high toughness, and weldability are required and provides a method for inexpensively producing that steel pipe without tempering.

The inventors studied combinations of chemical ingredients and the cooling rate and stop temperature of accelerated cooling for producing the optimum structure enabling achievement of both high strength and high toughness across the entire surface in the plate thickness direction even in an environment where differences occur in the cooling rate at the outside surface and inside surface due to accelerated cooling only from the outside surface.

The present invention was made based on the discoveries obtained by these studies and has as its gist the following:

(1) A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass %, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further having one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.

(2) A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in (1), characterized in that in said metallurgical structure, the average size of areas surrounded by high angle boundaries with an orientation difference of 15° or more is 30 μm or less.

(3) A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in (1) or (2), characterized in that in said metallurgical structure, the average particle size of the cementite is 400 nm or less and the density is 2×10⁵/mm² or more.

(4) A method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in any one of (1) to (3), said method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by forming a steel billet comprised of steel having the chemical ingredients as set forth in (1) into a pipe by hot piercing, rolling, and elongation steps and cooling by acceleratedly cooling the obtained steel pipe from the outer surface of the steel pipe from a temperature of 750° C. or more until a temperature T (° C.) satisfying the following equation (1) by a cooling rate V (° C./s) of 5 to 50° C./s while rotating the pipe in the circumferential direction, and subsequently by air cooling:

150<T<821.34×V ^(−0.3112)  (1)

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors first studied the metallurgical structure and form of cementite of a seamless steel pipe or machine structural use produced by the prior art quenching and tempering and the effect on the strength and toughness and obtained the following discovery.

When using quenching-tempering to produce steel pipe, cementite precipitates in the matrix at the time of quenching and the residual austenite breaks down into cementite and ferrite at the time of tempering. The thus obtained tempered martensite structure has cementite of an average particle size of 500 nm or more and is inferior in the balance of strength and toughness (below, called the “strength-toughness balance”).

Next, the inventors postulated a process of production of seamless steel pipe by accelerated cooling without tempering and studied metallurgical structures for improving both the strength and toughness and production conditions for obtaining the same.

As a result, they obtained the discovery that by making the metallurgical structure of the steel a structure in which cementite is made to finely precipitate in the grains, in particular self-tempered martensite or self-tempered martensite containing lower bainite, by optimizing the steel composition and accelerated cooling conditions, the strength-toughness balance is improved.

Furthermore, the inventors investigated in detail the relationship between the form of cementite and the strength and toughness and as a result learned that if the average particle size is 400 nm or less and the density is 2×10⁵/mm² or more, an extremely good strength-toughness balance is obtained.

Further, from the viewpoint of the strength-toughness balance, it is learned that control of the particle size is important. In the present invention, a crystal orientation map of steel having a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite is prepared by an electron back scattering pattern (EBSP) and the strength-toughness balance was investigated.

As a result, it was learned that the strength-toughness balance is improved when the average particle size of areas surrounded by high angle boundaries with an orientation difference of 15° or more (hereinafter also referred to as a “high angle boundary average size”) is 30 μm or less.

Below, the present invention will be explained in detail.

In the present invention, the reasons for limiting the chemical ingredients of the steel pipe will be explained. Note that the “%” shown below means “mass %” unless otherwise indicated.

C: C is an element extremely effective in improving the strength. To obtain the target strength, at a minimum 0.03% is necessary. However, if containing 0.1% or more of C, the low temperature toughness remarkably falls and further the cracking at the time of welding becomes a problem. Therefore, C is limited to 0.03 to less than 0.1%.

Mn: Mn is an element essential for improving the balance of strength and low temperature toughness. The lower limit is 0.8%. However, if greater than 2.5%, conversely the low temperature toughness greatly deteriorates, so 2.5% was made the upper limit.

Ti: Ti not only forms fine TiN and makes the structure finer, but also increase the hardenability and the toughness. If less than 0.005%, the effect is small, so the lower limit was made 0.005%. However, if greater than 0.035%, coarse TiN and TiC precipitate and the low temperature toughness remarkably falls, so the upper limit was made 0.035%.

Nb: Nb not only suppresses recrystallization of the austenite at the time of rolling and increases the fineness of the structure, but also increases the hardenability and increases the toughness of the steel. If less than 0.003%, the effect is small, so the lower limit was made 0.003%. However, if more than 0.04%, coarse Nb precipitates are formed and the toughness deteriorates, so the upper limit was made 0.04%.

B: B is an element increasing the hardenability and increasing the toughness. The lower limit where the effect is obtained is 0.0003%. On the other hand, if greater than 0.003%, conversely the hardenability falls. Some ferrite is produced and the target strength cannot be satisfied, so the upper limit was made 0.003%.

The upper limits of the deoxidizing elements Si and Al and the impurities P, S, and N are as follows:

Si: Si is a deoxidizing element, but if excessively added, the low temperature toughness is impaired, so the upper limit was made 0.5%. When adding Al as the deoxidizing element, there is no need to add Si and the lower limit may be made 0% as well.

Al: Al is a deoxidizing element, but if excessively added, coarse Al oxides are produced, low temperature toughness is invited and further the weldability is impaired, so the upper limit was made 0.05%. When adding Si as a deoxidizing element, there is no need to add Al and the lower limit may be made 0% as well.

P: P is an impurity and lowers the toughness, so the upper limit was made 0.015%. From the viewpoint of securing the toughness, the content of P is preferably 0.01% or less.

S: S is an impurity and lowers the toughness, so the upper limit was made 0.008%. From the viewpoint of securing the toughness, the content of S is preferably 0.005% or less.

N: N is an impurity. If more than 0.008%, coarse TiN is formed and upper bainite is formed and the toughness is impaired, so the upper limit was made 0.008%. Note that N forms TiN and other fine nitrides and sometimes contributes to increasing fineness of the structure, so 0.001% or more should be contained.

Further, one or more of Ni, Cr, Cu, and Mo may be added.

Ni: Ni is an element improving the strength. 0.1% or more is added. However, if over 1.5%, the element unevenly precipitates and the structure becomes uneven and the toughness sometimes deteriorates, so the upper limit is made 1.5%.

Cr: Cr is an element improving the strength. 0.1% or more is added. However, if over 1.5%, conversely Cr precipitates are formed and the toughness sometimes deteriorates, so the upper limit is made 1.5%.

Cu: Cu is an element improving the strength. 0.1% or more is added. However, if over 1.0%, upper bainite is formed and the toughness is sometimes impaired. Further, the weldability sometimes deteriorates, so the upper limit is made 1.0%.

Mo: Mo is an element contributing to higher strength. To obtain the effect of improvement of the hardenability, 0.05% or more is added. However, if over 0.5%, the weldability is sometimes impaired, so the upper limit is made 0.5%.

Next, the metallurgical structure will be explained. The metallurgical structure of the steel of the present invention is a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite. The self-tempered martensite and lower bainite are structures obtained by accelerated cooling. Due to these structures, it is possible to obtain an excellent balance of strength and toughness without tempering.

When acceleratedly cooling steel pipe from the outer surface, at the inside surface, the cooling rate becomes slower than the outside surface, and bainite and other high temperature transformation phases are easily formed. Further, when the plate thickness is great, if cooling so that the inside surface cooling rate becomes larger, the steel pipe sometimes deforms, so it is necessary to control the cooling rate to an extent where the steel pipe does not deform.

In this case, bainite transformation sometimes occur at the inside surface side of the steel pipe, but if lower bainite, a strength-toughness balance can be secured, so there is no particular problem. However, in machine structural use steel pipe, to make the entire surface in the plate thickness direction lower bainite, it is necessary to add a large amount of Mo. The economicalness is sometimes impaired.

Therefore, the metallurgical structure of the steel has to be a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.

Note that in the present invention, self-tempered martensite means the structure resulting from the austenite phase transforming to martensite during accelerated cooling and fine cementite precipitating in the laths by the gradual cooling after the accelerated cooling is stopped. The structure obtained by normal tempering is tempered martensite. Compared with this, the cementite of self-tempered martensite is extremely fine.

Further, in the present invention, lower bainite is defined as the structure resulting from lath type ferrite forming during accelerated cooling and fine cementite precipitating in one direction in the laths. Self-tempered martensite and lower bainite are common in the points that there is no coarse cementite at the grain boundaries and there is fine cementite in the matrix.

Self-tempered martensite and lower bainite are both lath type states, but these can be differentiated by the state of precipitation of cementite in the laths. That is, there are several long axis directions of cementite in self-tempered martensite, while lower bainite has a single long axis direction of cementite.

The self-tempered martensite and lower bainite defined in the present invention will be explained below as to their points of difference from other structures.

Upper bainite is a structure resulting from acicular cementite or a martensite-austenite mixed structure formed at the lath boundaries. Ferrite is not lath shaped such as bainite, but is aggregate in form. Pearlite is comprised of plate shaped cementite precipitated at the grain boundaries.

Self-tempered martensite and lower bainite can be judged if using a scanning electron microscope (SEM) for observation by a power of 2000× to 50000×. The sample should be mirror polished at its observed surface and etched by Nital.

Further, the average particle size of areas surrounded by high angle boundaries with an orientation difference of 15° or more has an effect of propagation of cracks at the time of breakage. If the high angle boundary average size becomes 30 μm or more, the toughness falls, so from the viewpoint of the strength-toughness balance, the high angle boundary average size is preferably 30 μm or less.

The smaller the high angle boundary average size, the better the strength-toughness balance, but with the current art, reduction to 3 μm or less is difficult. Note that the high angle boundary average size can be found from a crystal orientation map measured by EBSP.

The average particle size of the cementite is preferably 400 nm or less. This is because if the average particle size of the cementite is over 400 nm, the toughness falls. The smaller the average particle size of the cementite, the better, but cementite finer than 30 nm is difficult to judge by SEM, so in the present invention, the upper limit of the average particle size of cementite with a particle size of 30 nm or more is prescribed as 400 nm.

Further, if the density of the cementite is 2×10⁵/mm² or more, there is almost no formation of coarse cementite and an extremely good strength-toughness balance can be obtained. The upper limit of the density of the cementite is not particularly limited, but is determined by the amount of addition of C and average particle size.

Next, the method of production will be explained. In the present invention, the conditions when acceleratedly cooling steel pipe having the above chemical ingredients from a temperature of 750° C. or more are important. Note that the cooling rate is that at a position of the inside surface of the steel pipe.

The accelerated cooling stop temperature is one root technology of the present invention. The reason is that this has a major effect on the precipitation behavior of cementite in the matrix—which is the most effective for improvement of the strength-toughness balance.

If acceleratedly cooling the pipe by the above cooling rate V (° C./s) to cause martensite transformation and stopping the accelerated cooling at the temperature T (° C.) shown in the following formula (1), the subsequent air cooling causes fine cementite to precipitate in the matrix and enables self tempered martensite to be obtained.

On the other hand, if the accelerated cooling stop temperature becomes 150° C. or less, the subsequent air cooling will not cause cementite to precipitate and self tempered martensite cannot be obtained. Therefore, the lower limit was made less than 150° C.:

150<T<821.34×V ^(−0.3112)  (1)

Next, the range of the cooling rate in the accelerated cooling of the steel pipe will be explained. If the cooling rate is less than 5° C./s, upper bainite and ferrite are formed, while if over 50° C./s, uniform cooling becomes difficult and, after cooling, the steel pipe greatly deforms. Therefore, the accelerated cooling rate was limited to 5 to 50° C./s. Note that when the cooling rate is made that range, when the cooling rate is slow, lower bainite easily forms.

The reason for limiting the temperature for starting the accelerated cooling of the steel pipe to 750° C. or more is to make the metal structure at the time of start of accelerated cooling an austenite single phase. If the temperature of the steel pipe when starting the accelerated cooling is too high, the austenite grains will become coarser and a drop in toughness will be invited, so the accelerated cooling start temperature is preferably 950° C. or less.

The cooling start temperature and cooling stop temperature of the inside surface of the steel pipe should be measured before and after the accelerated cooling, that is, at the inlet side and outlet side of the cooling apparatus, by a contact thermometer at the inside surface of the steel pipe. It is possible to calculate the cooling rate from the temperature difference and the rate of passage through the cooling apparatus. It is also possible to measure the temperature of the outside surface of the steel pipe by a radiant thermometer and find the temperature of the inside surface of the steel pipe by calculation of the heat conduction.

Further, it is possible to attach thermocouples to the inside surface and the outside surface of steel pipes having various outside diameters and thicknesses, prepare cooling curves corresponding to various heating temperatures, refrigerant spraying conditions, and cooling times, and determine the conditions giving the range of the present invention.

The steel pipe of the present invention is seamless steel pipe. The pipe-forming process is generally hot piercing, rolling, and elongation, but it is also possible to use cold machining for piercing, then heat the steel and produce the pipe by a hot extrusion press. Further, diameter reduction rolling is also possible.

After the steel pipe production process is finished, the steel pipe may be raised in temperature by a heating furnace or induction heating. If the temperature of the steel pipe is 750° C. or more after forming a steel billet into pipe by hot piercing, rolling, and elongation, accelerated cooling as is in-line is also possible.

The method of accelerated cooling is limited to the method of cooling from only the outside surface while rotating the steel pipe in the circumferential direction. Due to this, it is possible to uniformly cool the pipe across the circumferential direction and the longitudinal direction.

On the other hand, if not rotating the steel pipe, the bottom surface of the steel pipe will excessively cool. Further, if cooling from the inside surface side, there is the problem that the water will build up at the bottom surface and the cooling rate will not become uniform.

For the cooling method, any of the method of bringing water into direct contact with the outside surface of the steel pipe, the method of bringing it into contact with the tangential direction of the outer circumference of the steel pipe, mist cooling, etc. may be selected.

In the present invention, the steel pipe shape which can be applied is preferably made a shape of a length of at least five times the outside diameter. This is because when the length is less than five times the outside diameter, when performing the accelerated cooling from the outside surface by water cooling, water penetrates to the inside surface of the steel pipe as well resulting in uneven cooling and bending of the steel pipe.

Note that for reliable, uniform accelerated cooling, the length of the steel pipe is preferably made at least 10 times the outside diameter.

EXAMPLES

Steels of the chemical ingredients shown in Table 1 were produced and cast into blooms of a diameter of 170 mm by a converter-continuous casting process. In Table 1, blank cells indicate the analyzed values of the ingredients were less than the detection limits.

These blooms were heated to 1240° C., pierced and rolled by the Mannesmann plug mill process, then reheated to 950° C., rolled to reduce the diameter, then water cooled from the outside surface side by ring cooling by direct feed.

Further, some of the steel pipes were obtained by cooling reduced diameter rolled seamless steel pipes to room temperature, then reheating to 950° C., then water cooling from the outside surface by ring cooling.

There were three steel pipe sizes after diameter reduction rolling: outside diameter 126 mm and thickness 12.2 mm (small (S)), outside diameter 138 mm and thickness 16.4 mm (medium (M)), and outside diameter 146 mm and thickness 20.6 mm (large (L)). The length was 6.5 m in each case.

Test pieces were taken from the produced steel pipes at any positions of the circumferential direction, longitudinal direction, and thickness direction, buried in resin, mirror polished and etched, then observed for structure by an SEM at a maximum power of 50000×. The structures were classified into self tempered martensite (M), lower bainite (LB), upper bainite (UB), and ferrite (F).

Further, 10 SEM structure photographs of powers of 10000 to 50000× were used for image analysis to find the average value of the circle equivalent radius of cementite and number per unit area (mm²).

Furthermore, the metallurgical structure was observed by an optical structure and measured for Vicker's hardness at 10 kgf based on JIS Z 2244.

Further, the surface of each sample buried in resin was electrolytically polished. Using an EBSP mounted on an SEM, the crystal orientation was measured. The grain boundaries having orientation differences of 15° or more were identified, the average value of the circle equivalent radius of the area surrounded by the grain boundaries was found by image analysis, and the result was indicated in the column of high angle boundary average size of Table 2.

A tensile test was performed using a No. 11 test piece of JIS Z 2201 based on JIS Z 2241 to measure the yield strength and tensile strength. The toughness was evaluated by running a Charpy impact test based on JIS Z 2242 using a 2 mmV notch full size test piece at −40° C. and measuring the absorption energy (vE₋₄₀ (J)).

The weldability was evaluated by welding steel pipes together at room temperature using welding wire having a 780 MPa class strength by CO₂ gas welding to prepare steel pipe joints, inspecting for the presence of cracks by visual inspection after 24 hours, then judging ones with no cracks as passing.

After accelerated cooling, the shapes (bending) of the steel pipes were measured at room temperature. Steel pipes of lengths of 6.5 m were brought into contact with flat plates at the end of one side, a location of a distance from the end of 0.5 m, and a location of such a distance of 1 m, that is, a total of three locations, the steel pipes were rotated, and the maximum rise of the end of the steel pipe at the opposite side.

The maximum rise of the end of the steel pipe is the height of the bottommost part of the raised end of the steel pipe from the flat plate. Steel pipes with a maximum rise of the end of not more than 10 mm were judged as passing in steel pipe shapes.

TABLE 1 Steel Chemical ingredients (mass %) type C Si Mn P S Al Ti Nb N B Ni Cr Cu Mo Remarks A 0.060 0.13 2.00 0.008 0.003 0.029 0.012 0.021 0.0057 0.0011 0.45 0.30 Inv. ex. B 0.090 0.13 1.50 0.015 0.005 0.043 0.025 0.017 0.0023 0.0015 0.30 0.20 0.40 Inv. ex. C 0.035 0.39 2.70 0.014 0.005 0.031 0.015 0.032 0.0059 0.0008 0.10 0.10 0.10 0.20 Inv. ex. D 0.055 0.23 2.20 0.007 0.002 0.022 0.021 0.037 0.0060 0.0012 0.80 0.40 Inv. ex. E 0.060 0.43 1.88 0.009 0.007 0.020 0.025 0.009 0.0064 0.0004 0.10 1.10 Inv. ex. F 0.085 0.30 0.85 0.009 0.003 0.020 0.009 0.017 0.0068 0.0018 0.30 0.80 0.30 Inv. ex. G 0.048 0.43 1.70 0.010 0.006 0.034 0.012 0.038 0.0027 0.0027 0.83 0.22 Inv. ex. H 0.052 0.25 1.30 0.007 0.005 0.044 0.023 0.012 0.0051 0.0027 0.70 0.45 0.35 Inv. ex. I 0.072 0.26 1.78 0.014 0.003 0.043 0.019 0.036 0.0058 0.0029 0.50 0.20 Inv. ex. J 0.032 0.34 2.80 0.011 0.004 0.022 0.015 0.032 0.0043 0.0002 1.45 Inv. ex. K 0.045 0.38 1.55 0.014 0.004 0.044 0.014 0.037 0.0055 0.0013 1.30 Inv. ex. L 0.062 0.36 1.40 0.010 0.007 0.026 0.019 0.035 0.0029 0.0026 1.00 Inv. ex. M 0.098 0.19 0.95 0.013 0.006 0.033 0.016 0.020 0.0067 0.0005 0.50 Inv. ex. N 0.150 0.22 0.85 0.112 0.007 0.024 0.017 0.034 0.005 0.0050 1.80 0.10 Comp. ex. O 0.022 0.18 3.50 0.166 0.007 0.038 0.050 0.008 0.007 0.0013 0.05 0.05 0.05 0.03 Comp. ex. P 0.060 0.29 0.65 0.230 0.002 0.031 0.019 0.001 0.003 0.0008 1.80 0.30 0.15 Comp. ex. Q 0.055 0.72 1.80 0.062 0.004 0.033 0.003 0.012 0.007 0.0005 Comp. ex. R 0.058 0.25 1.65 0.136 0.012 0.024 0.012 0.013 0.097 0.0002 0.30 1.50 Comp. ex. S 0.062 0.21 1.85 0.066 0.001 0.068 0.025 0.052 0.006 0.0029 0.30 0.80 Comp. ex.

The results are shown in Table 2. The underlines in Table 2 mean outside the range of the present invention or outside the preferable range. The invention examples of No. 1 to 13 are steel pipes produced under suitable accelerated cooling conditions and are provided with suitable metal structures and the strength and toughness required as machine structural use steel pipes.

No. 14 is an example which was high in amount of C, amount of B, and amount of Ni and was high in accelerated cooling stop temperature, so an upper bainite structure was formed, the toughness fell, and the weldability fell.

No. 15 is an example which was too low in amount of C, insufficient in hardenability, and high in accelerated cooling stop temperature, so a partial upper bainite structure formed, the toughness fell, and the cooling rate was fast, so the shape also deteriorated.

No. 16 is an example where the amount of P was particularly high and the accelerated cooling start temperature was low, so ferrite was formed and the toughness fell.

No. 17 is an example where the amount of Si was too high, upper bainite was formed, and the toughness became poor. No. 18 is an example where the amount of N, amount of Cu, and amount of S were too high and the cooling rate was slow, so upper bainite was formed, the toughness was impaired, and the weldability dropped.

No. 19 is an example where the amount of Al and amount of Nb were too high and the accelerated cooling stop temperature was high, so upper bainite was formed and the toughness was impaired and where Al was excessively included, so the weldability was also poor.

No. 20 was an example which was too slow in cooling rate, while Nos. 21 and 25 were examples which were too high in accelerated cooling stop temperature, so upper bainite was formed and the toughness fell.

Nos. 22 and 24 are examples where the cooling rate is too fast and the accelerated cooling stop temperature was high, so a mixed structure of tempered martensite and upper bainite resulted, the toughness was low, and the shape was also poor.

No. 23 is an example where the accelerated cooling start temperature was too low and ferrite was formed, so the toughness was poor.

TABLE 2 Metal structure Accelerated Cooling Accelerated High angle Cementite cooling start rate cooling stop boundary average Cementite Steel temperature V temperature average size particle particles × 1O⁵ No. type Size Process (° C.) (° C./s) (° C.) 821.34 × V^(−0.3112) Class (μm) size (nm) (/mm²) 1 A S Direct feed 949 33.9 207 274 M 20.7   142 5.5 2 B S Direct feed 996 27.0 165 294 M 27.5   114 12.3  3 C S Direct feed 875 32.0 239 279 M 26.1   199 2.4 4 D S Direct feed 819 18.0 285 334 M + BL 17.5   211 3.9 5 E S Reheat 957 15.2 283 352 M + BL 24.6   91 8.8 6 F M Reheat 861 23.9 294 306 M + BL 14.8   93 15.3  7 G M Direct feed 865 19.6 152 325 M 29.4   345 47.3  8 H M Direct feed 812 38.4 211 264 M 26.6   277 22.8  9 I M Direct feed 907 11.0 279 389 M + BL 23.2   332 32.7  10 J M Direct feed 999 46.6 171 248 M 19.7   330 70.0  11 K L Reheat 913 18.3 187 333 M 27.2   279 11.8  12 L L Reheat 764 31.4 157 281 M 16.7   279 3.2 13 M L Direct feed 813 18.0 255 334 M + BL 10.3   171 59.0  14 N S Direct feed 848  8.0 500 430 M + BU 65.2 2,100 0.9 15 O S Direct feed 886 58.0 350 232 M + BU 10     520 0.5 16 P M Reheat 680 48.2 178 246 F + M 12.5   199 3.9 17 Q M Reheat 942 41.1 222 258 M + BU 29.4   555 1.2 18 R L Direct feed 893  6.0 378 470 M + BU 45     820 0.8 19 S L Direct feed 925 45.1 308 251 M + BU 13.7   423 2.1 20 A S Reheat 1100   6.2 222 466 M + BU 53.5   770 1.1 21 A S Reheat 949 32.4 378 278 M + BU 22.7   466 5.5 22 A M Direct feed 787 65.0 410 224 M + BU 48.2   980 1.5 23 A M Direct feed 670 32.0 188 279 F + M 21.5   244 6.3 24 A L Reheat 941 57.3 333 233 M + BU 12     620 3.3 25 A L Reheat 919 46.4 475 249 M + BU 45.3  1200 0.8 Mechanical proper ties Tensile strength Yield strength Shape of No. (MPa) (MPa) vE⁻⁴⁰ (J) Weldability steel pipe Remarks 1 972 785 265 Good Good Inv. ex. 2 1085 837 211 Good Good Inv . ex. 3 912 747 299 Good Good Inv. ex. 4 955 841 265 Good Good Inv. ex. 5 965 815 242 Good Good Inv. ex. 6 1036 848 236 Good Good Inv . ex. 7 909 724 277 Good Good Inv. ex. 8 1036 812 247 Good Good Inv. ex. 9 1013 840 230 Good Good Inv. ex. 10 926 726 286 Good Good Inv. ex. 11 954 758 230 Good Good Inv. ex. 12 1018 789 256 Good Good Inv. ex. 13 972 822 299 Good Good Inv. ex. 14 1150 920 36 Poor Good Comp. ex. 15 880 810 18 Good Poor Comp. ex. 16 1150 805 12 Good Good Comp. ex. 17 810 664 45 Good Good Comp. ex. 18 860 760 24 Poor Good Comp. ex. 19 820 730 68 Poor Good Comp. ex. 20 780 624 22 Good Good Comp. ex. 21 968 812 108 Good Good Comp. ex. 22 864 812 55 Good Poor Comp. ex. 23 1058 745 23 Good Good Comp. ex. 24 111 913 111 Good Poor Comp. ex. 25 815 730 78 Good Good Comp. ex.

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to provide a high strength seamless steel pipe for machine structure use superior in toughness and weldability suitable for a machine structural member, in particular, a cylinder, bushing, boom, or other structural member and shaft or other machine member and a method inexpensively producing the steel pipe. Therefore, the present invention contributes to industry extremely remarkably. 

1. A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass %, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further having one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.
 2. A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in claim 1, characterized in that in said metallurgical structure, the average size of areas surrounded by high angle boundaries with an orientation difference of 15° or more is 30 μm or less.
 3. A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in claim 1 or 2, characterized in that in said metallurgical structure, the average particle size of the cementite is 400 nm or less and the density is 2×10⁵/mm² or more.
 4. A method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in claim 1, said method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability, characterized by forming a steel billet comprised of steel having the chemical ingredients as set forth in claim 1 into a pipe by hot piercing, rolling, and elongation steps and cooling by acceleratedly cooling the obtained steel pipe from the outer surface of the steel pipe from a temperature of 750° C. or more until a temperature T (° C.) satisfying the following equation (1) by a cooling rate V (° C./s) of 5 to 50° C./s while rotating the pipe in the circumferential direction, and subsequently by air cooling 150<T<821.34×V ^(−0.3112)  (1). 